1. Field of the Invention
The present invention relates to a semiconductor device using a semiconductor element such as a transistor, and more specifically relates to a semiconductor device provided with a protection means in order to prevent electrostatic discharge damage.
2. Description of the Related Art
Static electricity builds up, developing due to friction, contact and the like with air and objects such as conductors, semiconductors and insulators. When there are an excess of charges on these materials, electrostatic discharge is caused, and in the case where this condition occurs on a free node such as an input terminal, minute semiconductor elements formed on a substrate are degraded or destroyed. This is referred to as electrostatic discharge damage. In order to prevent such electrostatic discharge damage, a circuit formed on a substrate (hereinafter referred to as an internal circuit) is connected to an external IC (hereinafter referred to as an external circuit) via a protection circuit and an FPC. The protection circuit detects voltage and current supplied from the external circuit to the internal circuit, and controls the voltage and current to prevent the internal circuit from being damaged when a malfunction occurs.
In recent years, a semiconductor device including a light emitting element has been actively developed. This semiconductor device has the features such as fast response, superior dynamic display and wide viewing angle, as well as the advantages of a conventional liquid crystal display device. The semiconductor device comprises a plurality of pixels each including a light emitting element and a transistor, and in each of the pixels, the transistor connected in series to the light emitting element controls light emission or non-light emission of the light emitting element.
As a transistor for driving the light emitting element, a crystalline semiconductor (polysilicon) is preferably used because of the high field effect mobility. A transistor using polysilicon, however, tends to have variations in characteristics due to defects in crystal grain boundaries. Accordingly, the drain current of the transistor differs in each pixel even when the same signal voltage is inputted, leading to variations in luminance. In order to reduce such variations in luminance, a driving transistor may be operated in a saturation region to supply a drain current corresponding to a voltage VGS between a gate and a source (see Patent Document 1 for example).
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-108285